Beginners Guide to riding Fixed/Single Speed in London

Again though.....running bags aren't designed to carry big loads...if a running bag was carrying the same weight as a messenger bag is capable of (which it can't) it would bounce when leaned over on a bike.
As i said....two different things completely.

Just trying to illustrate that the two bags are designed to resist different types of movement.

Decent running bags are incredibly well designed. TBH I feel its a waste of clever engineering cycling with them. Mine has a certain amount of elasticity built into the straps which is great for absorbing bounce from the running motion, but would be a PITA on a bike (especially with any weight in it).

My bike rucksack (MTN bike bag) cleverly has the connecting strap (the one that connects the shoulder straps) lower than usual (just under the ribs), to allow arm movement when cycling. This means that the weight is poorly supported when you stand up straight, which would be a PITA when running.

Horses for courses.